Neurogenesis continues in selected regions of the adult mammalian brain and generates new neurons that integrate into existing neural circuitry. In addition to the two well characterized neurogenic niches -the subventricular zone (SVZ) of the striatum and the subgranular zone of the hippocampal dentate gyrus - the olfactory epithelium (OE) also harbors stem cells that produce olfactory sensory neurons postnatally. However, the molecular identity of the OE stem cell is as yet unsettled. Moreover, the multipotency of the OE stem cells in vivo and in vitro is incompletely characterized. Lastly, the plasticity of these stem cells is unknown. The overall objective of this application is to molecularly identify and purify the OE stem cells and to functionally characterize them in vivo and in vitro. We hypothesize that Inhibitor of DNA-binding/differentiation (Id) protein expression marks the OE stem cells, based on the following: (1) These dominant-negative inhibitors of basic-helix-loop-helix transcription factors inhibit differentiation and promote self-renewal in stem cells;(2) Id1 mRNA is present in few cells in the basal layer of the OE, and is up-regulated by olfactory bulbectomy;(3) clusters of basal cells express Id1 protein, and some of these cells are in S-phase in vivo;(4) when purified, these Id1+ cells can proliferate in vitro. The specific aims of this application are (1) to further characterize the antigenic and molecular signature of Id1+ cells in the OE by immunofluorescence and microarray analyses;(2) to determine whether the Id1+ cells from the OE can self-renew in vitro and in vivo by neurosphere and transplantation assays;(3) to determine whether the Id1 + cells are multipotent in vitro and in vivo and what cell types are generated by differentiation and transplantation assays. The study would provide a novel insight into the molecular identity of the OE stem cell and form the foundation for future studies. In the long-term, this line of investigation could shed light on stem cell-to-cell and cell-to-niche interaction, mechanism of stem cell self-renewal, adult stem cell plasticity, as well as age-related decline in physiological stem cell function. These may be directly or indirectly relevant to translating the stem cells into clinically applicable cell-based therapeutics for neurodegenerative disorders, e.g., Parkinson's disease. In addition, as many as 50% of patients over 65-years-old may be affected in their sense of smell. The age-related decline in olfactory function likely affects the patient's quality-of-life, well being, and personal safety. An insight into molecular and cellular changes that occur in the OE stem cells and niche may suggest avenues for rationally designed therapeutics.